About Me

Monday, February 27, 2012

Back in about 2003 I was asked to design a rowing/small outboard boat for operations in a short, steep chop, with the emphasis on lack of pounding. The resulting boat, named Flint, has been very successful and popular. She is very easy to build - if you follow the assembly sequence - and a distictive feature of her hull is the fine, hollow sections in the forward part of the boat. What is amazing is that this shape can be produced from a single developed plywood panel. I have tried to achieve this sort of shape in the past using conventional projection of developable panels, but the geometry required to produce a hollow shape has always defeated me. The arrival of computer programs which have the ability to do the mathematics saved me from myself!

Proof that the hollow sections come from a single panel of plywood

The very first Flint, built by Eddie Guy

In recent times I have been approached by a number of people who have wanted to build a planing version of Flint. The hull of the original rowing/sailing/small outboard version is of the displacement type, although her fine sections and high length-to-breadth ratio mean that she can be pushed faster than her theoetical displacement speed which is 5 knots. On launching day we got her to 6.1 knots with two adults and two teenagers onboard, using a 2hp Yamaha at part throttle. Steve Dorrington got his to a speed of 6.2 knots under the urge of British Seagull 40-plus - certainly an achievement!

Steve Dorrington measuring his 6.2 Seagull-knots using a GPS. Note how Steve has to sit on the midships thwart in order to retain proper trim. In the case of the planing version, a helms-person should be able to sit aft and operate the outboard without a tiller extension.

Bruce Erney, on the east coast of the USA modified his Flint to take a 6hp motor and got her up to 17 knots, I think, but he needed to put trim tabs on to overcome the tendency of the displacement hull to point her bow at the sky. I was concerned about the exercise, but Bruce had many successful trips while fishing in the big waters.

Bruce Erney's trim-tabs

One of Bruce's many fishing trips in his modified Flint

I've made a number of attempts at drawing a planing version of Flint and things were brought to a head this week when a friend/customer for whom I have great respect, asked me for a semi-disposable planing power boat which he and a friend could build over a weekend or two, to be powered by a 5hp outboard. He looked at few drawings I had on hand, and chose a preliminary sketch of a planing version of Flint, which I drew as a discussion piece for Rick Hayhoe a number of months ago.

Lines Drawing of the original Flint

Lines drawing of the planing version, now called Fleet

In the above drawings you can see the substantial changes made to the shape of the hull aft of the midsection. I have retained the fine, hollow forward sections, although I've increased the rake of the stem very slightly in order to reduce the severity of the bending and twisting of the plywood panels. With good-quality ply the hull is easy to assemble, but some people still insist on using poor-quality sheets in order to save a dollar, and end up encountering problems - hence the alteration.

My aim with this design is to retain the extemely light and simple style of hull as seen with the original Flint, and to have her run efficiently with nothing larger than a 4hp motor. This is important in my part of the world because we can operate with up to, and including, 4hp without needing to register the boat. The customer wants to use a 5hp, and specifically asked for a water-shedding foredeck and side decks under which he can store fishing gear. He intends taking her into the open Pacific off the Queensland coast here in Australia, and so adequate emergency floatation is essential. His boat will have built-in buoyancy under the sternsheets (aft seating which extends aft in a horseshoe arrangement), under the midships thwart, and under the large foredeck. We have not finalised details of the arrangement, but here is a very preliminary sketch with the decks and seating shown in blue.

Here is a very sketchy drawing showing one idea of an internal layout. The final version is a fair way off yet, I think. The decks have very pronounced camber to aid in water shedding (those odd looking bits at the bottom are just working drawings to establish the height of the inner edges of the side decks at each station). Note how the cambered foredeck makes the sheer look much flatter than in the undecked original - this is an optical illusion.

This little article should give some indication of just how much the character of a boat must be changed when the mode of operation is altered. However, a completely open version with a very small motor may retain the spartan feel of the original, while still performing in the semi-planing and planing modes. When the plans are complete and the boat tested, I'll post more details. For those who want to read more about the original Flint follow this link and this link.

Sunday, February 19, 2012

Back in December 2011 there was some animated discussion about the design of boats with a transom bow - in this instance the discussion related to my Whimbrel design - with a number of people expressing their concern about Whimbrel's bow transom being too wide and too close to the waterline. The feeling seemed to be that the bow would dig in on some points of sail, and would bash into the wave in a steep, choppy head sea, therefore slowing the boat, throwing spray into the air, and maybe causing wild steering running downwind.

This perspective shows Whimbrel's hull viewed from the starboard bow - you can see that the base of the bow transom is close to the waterline when the boat is heavily loaded.

In this drawing there are three different waterlines drawn - the deepest one with her at close to one ton of displacement, which would not be likely to occur. But even at that heavy displacement the heel of the transom is just above the static waterline.

My argument was (and is) that the boat was designed to be only seventeen feet long, and that she would be faster, roomier, and more stable - for the load-carrying ability specified - with the large bow transom. For some background discussion on this matter have a look at this post . In fact, if I had been brave enough to withstand the negative comments, I would have made Whimbrel's bow transom vertical, as I am convinced that her performance would be superior if drawn that way. I must say the raked transom looks better in profile, but I'm sure the performance is degraded.

I often quote Phil Bolger in my discussions, and at the time of the Whimbrel debate I used one of his remarks about bow transoms.

Fieldmouse - scanned from a photocopy sent to a customer by Phil Bolger in October 1971. Note how the waterline is at the heel of the vertical bow and stern transoms.

Here is Phil's quote when discussing his Supermouse and Fieldmouse designs: -

"Raked transoms make a faster boat less badly stopped by waves if they are raked out from the given bottom length, but in that case the boat would be better still if the waterline were carried out to plumb transoms at the new overall length...." (Boats with an Open Mind, International Marine 1994)
The logic in this remark seems to be lost on most people, but the technical importance of this comment, and others, has obviously been understood by commenter, Graeme: -

"No corners painted by Bolger here - even on those square boats! ;-)

What's not to understand? Drawing the bottom (and waterline) out to be as long as the deck of an otherwise raked ends hull makes for a narrower stern and narrower bow at the waterline. The finer entry is less stopped by waves and less wet. The drawn out waterline on the same beam has higher theoretical speed. Importantly (never forget the several intricate ways Bolger stressed the virtues of "shallow", this is but one) it also has less draft, and so less resistance, thereby being more likely to achieve or exceed that theoretical speed.

It is in no way all about the bottom though. What is often passed by in what Bolger said is that it's the decks that raise hull cog, and Bolger viewed decks, rightly, as a penalty. Other considerations not withstanding, he may just have stuck to raked ends, clipper bows, and such, if he'd been able to design decks of weightless unobtainium. As it is, plumb ends (and sides) impose a smaller deck weight penalty on a given bottom, or on stability, or on ability to stand up to sail, or on overall performance.

Flow on efficiencies there, which may also continue to run right into the pocket.

No corners then, painted or otherwise, rather Bolger's genious yet again seen remarkably squaring a circle unlooked for. A virtuous circle spiralling on with yet more angles round each turn.

Now, I am not saying that bow transoms are the answer to everything. Most of my designs have unusually fine, sharp sections up for'ard. In fact, I sometimes wonder whether I may overdo it in an attempt to cut down pounding in a short head sea.

The very sharp entry on Flint

Periwinkle has a long, fine entry.

What I am saying is that for certain applications, a hull with a bow transom may be the most appropriate way to design a hull to perform a particular function and if one of the design constraints is overall length, then a wide, vertical transom extending almost to the waterline should be considered.

Sunday, February 12, 2012

At the end of June 2011, I published a post about my idea for moveable side seating in cruising dinghies. The idea is that side seats can be used if prefered (although I like to sit down low in the boat with my weight near the windward turn of bilge) and can also be removed at a moment's notice. Most importantly, if they are slid into the centre of the boat they make an excellent sleeping platform, well above the bilge water.

The Phoenix III built by Paul Hernes, showing the side seats in the standard location..............

...........and slid together to form a nice sleeping platform............

I am continuing to work intermittently on a First Mate in my workshop - she gets attention when glue is curing on other jobs. Last week I installed the rails to carry the removable side-seats/sleeping platform, and I'm delighted to say that in First Mate, the seats are more than 26 inches wide at the upper part of the body when slid together. Here are a few pictures of the rails: -

Here is the seat rail glued and screwed onto the aft face of the midships frame. This frame is made from 12mm (1/2") marine plywood, with 38mm x 12mm (1-1/2" x 1/2") behind where the screws go through. The whole thing is very strong, and the seat rail would be fine with just a glue joint. However, the silicon bronze screws make it simple to position when the epoxy is slippery, so they may as well go in! The rails are positioned to carry a side seat which is 31mm (1-1/4") deep - these will most likely be 25mm (1") framing topped with 6mm (1/4") marine plywood.

A close-up of the aft rail, which attaches to the semi-bulkhead at the forward end of the stern seat (i.e. the "stern sheets"). The centre cut-away is just to allow positioning of the screw-in hatch to that particular buoyancy compartment.

Another shot of the aft seat rail with the rowing foot-braces on the floor below. These foot braces do double-duty as structural stiffening elements as well

Saturday, February 11, 2012

I'm sorry that it has been a few weeks since I last wrote a post, but I've been very busy with a whole range of activities - not just boat related - which have prevented me from sitting down to write. In addition I been feeling a bit blocked for things to write about. It isn't that there is a shortage of subject matter - it is just that depending on mood, I sometimes find it difficult to write in a way that I think is interesting to readers. I've been writing articles for every issue of a print magazine for the last twelve years, and now for this blog, and I can assure you that it isn't easy!

Anyway, here is a bit of what has been going on: -

Back in late October 2011, I mentioned that I was in the process of designing an "Egret-style" sharpie/dory for John Hockings in Brisbane, Australia. In early December 2011, I reported that John had commenced construction. Since then, John has been making very rapid progress and the boat is looking great. The photos have been taken with a relatively wide-angle lens so the boat looks smaller than she really is in the flesh.

Here is the hull viewed from the stern with gunwales, mast partners, mast steps, and centreboard case fitted. Side deck carlings are being sprung into place, with temporary cross-braces to hold them in against the twisting, bending, and edge-setting forces.

A photo from very early in the construction process which gives a good impression of the hull profile - bow is to the right of the picture.

A view of the interior during the application of a thinned epoxy (Norseal). The photo has been taken from the stern and you can see the mizzen mast location at the aft end of the centreboard case. The floor timbers (transverse frames on the bottom) will carry floor-boards in the finished boat, allowing passengers to sleep above any bilge water, and to give a nicely textured footing during normal sailing and poling. Note the seat risers which allow positioning of thwarts in many locations.

One topside panel cut and material for the second one waiting to be worked on. The panel shapes are precomputed and dimensioned drawings are supplied with the plans, allowing the boat to be built without any sort of mold or strongback.

View from the bow, with the first undercoat applied. Mast step in the foreground is for the third mast location, which will take either the main mast or the mizzen mast, allowing the boat to be sailed with a single sail if desired.

Another photo of the boat being undercoated, this time viewed from the stern with the main mast partner visible in the background.

This boat is an exciting prospect as far as I'm concerned. She will be rugged and yet is an easy project for any capable builder. John has been doing an exceptionally good job of building her and I really look forward to seeing how she handles the rough stuff as well as the thin water for which she is very well adapted.